Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into hydrocarbons.
This second step, the preparation of hydrocarbons from synthesis gas, is well known in the art and is usually referred to as Fischer-Tropsch synthesis, the Fischer-Tropsch process, or Fischer-Tropsch reaction(s). Fischer-Tropsch synthesis generally entails contacting a stream of synthesis gas with a catalyst under temperature and pressure conditions that allow the synthesis gas to react and form hydrocarbons. More specifically, the Fischer-Tropsch reaction is the catalytic hydrogenation of carbon monoxide to produce any of a variety of products ranging from methane to higher alkanes and aliphatic alcohols. Research continues on the development of more efficient Fischer-Tropsch catalyst systems and reaction systems that increase the selectivity for high-value hydrocarbons in the Fischer-Tropsch product stream.
The products of the Fischer-Tropsch synthesis may include a large range of molecular weights from light hydrocarbons such as methane to very large molecules with 50 or more carbon atoms. While hydrocarbon streams produced via Fischer-Tropsch synthesis may be used in a variety of applications, their use as liquid fuels is of significant interest. In particular, Fischer-Tropsch products are suitable for production of high cetane and low emissions diesel fuels. However, a significant portion of the products produced in the Fischer-Tropsch reaction are paraffin waxes that are heavier than the diesel boiling range specification and cause cold flow problems. Therefore, hydrocracking Fischer-Tropsch products is a common practice where diesel is the desired product.
Hydrocracking operations are generally controlled by monitoring the conversion of a select group of hydrocarbons. For example, it is possible to measure hydrocracker conversion by equation (1):
                    Conversion        =                                            C                                                21                  +                                ,                IN                                      -                          C                                                21                  +                                ,                OUT                                                          C                                          21                +                            ,              IN                                                          (        1        )            C21+,IN is the number of moles of hydrocarbons with 21 or more carbon atoms entering the reactor, and C21+,OUT is the number of moles of hydrocarbons with 21 or more carbon atoms exiting the reactor. This correlation is commonly used to control hydrocracker reactor severity. However, equation (1) does not provide compensation for the effect of feed composition on the cracking operation. Therefore, an improved method for controlling hydrocracking operations is needed.